1. Field of the Invention
The invention pertains to the field of porosimetery, or the measurement of the porosity of substances. More particularly, the invention pertains to a liquid extrusion method and apparatus for determining the porosity characteristics of a sample.
2. Description of Related Art
The prior art measures the pore volume of a sample using a weight and balance method. Specifically, as shown in FIG. 1, a sample (1) is wetted by water and then placed above a membrane (2). A reservoir of fluid (3) is located below the membrane (2). This fluid is the same type of fluid which is used to wet the sample (1). A vacuum is used to draw the liquid through the sample.
In vacuum systems, pressure cannot be controlled accurately or maintained at a constant value. The low pressure causes loss of liquid from pores due to evaporation.
The prior art uses water as the wetting fluid. Water has high air diffusivity, which increases the air bubbles in a sample, and potentially yields inaccurate results by increasing the volume of the displaced liquid. The viscosity of water is low, which also leads to bubble formation.
This equation below used for computing pore diameter (D) from results of porosimetry shows that accuracy of measurement is determined by surface tension, xcex3, and contact angle, xcex8, of the wetting liquid. For water xcex3 is large and changes easily due to contamination and xcex8 is also large and varies appreciably depending upon the nature of the sample. These uncertainties contribute to error.
D=4xcex3 cos xcex8/P
The sample (1) has larger pores (4) than the pores (5) of the membrane (2). Vacuum (13) is applied, until liquid is drawn out of the pores (4) in the sample, and into the reservoir of fluid (3). The displaced fluid (7) flows over the top of the reservoir container (8) and is caught in a receptacle (9). The receptacle (9) is on a balance (10), which weighs the amount of the displaced fluid (7). This weight change is used in combination with calculations known in the art to determine the volume of the pores (4) in the sample (1). A counterweight (11) on the balance (10) is used to determine the weight change due to the displaced fluid (7).
A porosimeter evaluates the porosity characteristics of a porous sample of material. The sample is preferably wetted, with the same type of fluid which is in the reservoir, prior to placing the sample on the porosimeter, or the fluid can be poured over the sample in the chamber and pressure applied to force the fluid into the pores of the sample.
The porosimeter of the present invention comprises a source of pressure connected to a pressurizable chamber for holding the sample, and a reservoir of fluid located below the sample, to which is connected a penetrometer comprising a tube into which fluid displaced from the reservoir of fluid can flow. Thus, the level of fluid will rise in the penetrometer when additional fluid enters the reservoir, and by measuring the level of fluid in the penetrometer the volume of fluid entering the reservoir can be measured.
The sample is supported by a membrane located between the sample and the reservoir of fluid. The membrane has a plurality of pores with a size smaller than any of the sample pores, so that the bubble point pore diameter of the membrane is smaller than the smallest pore of interest in the sample.
The pore volume of the wetted sample is determined by applying a pressure which is above the bubble point pressure of the sample, but below the bubble point pressure of the membrane, and measuring the change in fluid level in the penetrometer.
Preferably, a fluorocarbon or silicone liquid is used as the fluid in the porosimeter. Fluorocarbon and silicone liquids have low surface tension and the contact angle is zero for many materials. The low surface tension enables smaller pores to be measurable. Unchanging surface tension gives more accurate data. Zero constant contact angle gives more accurate and less uncertain results.
The same apparatus can be used to measure permeability of the sample by measuring flow versus time when pressure is applied to the sample.